1. Filed of the Invention
The invention relates to well logging with nuclear tools. In particular, it relates to apparatus and methods for the determination of formation properties using gamma-ray tools.
2. Background Art
The characteristics of geological formations are of significant interest in the exploration and production of subsurface water and mineral deposits, such as oil and gas. Many characteristics, such as the hydrocarbon volume, porosity, lithology, reservoir location, and permeability of a formation, may be deduced from certain measurable quantities. Among these quantities are: density, porosity, photoelectric factor (Pe), hydrogen index, salinity, and thermal neutron capture cross section (Sigma). These quantities are typically measured by logging-while-drilling (LWD) or wireline tools.
Some logging tools may detect signals originated from the formations without input of external energy. Other logging tools may carry a source that radiates or emits energy into the formation and one or more detectors that can sense the resulting interactions of the radiation. Detected signal data are typically transmitted uphole, temporarily stored downhole for later processing, or combined in both techniques, to evaluate the geological formation from which the data was gathered.
Natural gamma-ray measurements are performed with detectors that can measure the radioactivity of earth formations surrounding a borehole. A gamma-ray detector may comprise any suitable type of detector, such as NaI, BGO, CsI, anthracene, etc., see U.S. Pat. Nos. 4,647,781 and 4,883,956. In some cases, detailed analysis of the natural gamma-ray emission is desired so that contributions from the three principal naturally occurring radioactive substances: Thorium, Uranium and Potassium, can be separated. In such a case, the energy spectrum of the gamma-rays is measured either by the acquisition of several energy windows which are optimized to be mainly sensitive to one of the three isotopes or by acquiring a detailed spectral response through a multi-channel analysis. For example, a spectrum acquired by a NaI(Tl) scintillation spectrometer is shown in FIG. 1. This spectrum indicates a typical natural background, which comes from a mix of Th, U and K. Very little Th is visible in this spectrum, which appears to be dominated by U and K. In this spectrum, gamma-rays above 2600 keV are due to cosmic radiation.
Typically, the signals from natural radioactivity are very weak and easily biased by radiation from other sources. Currently, these measurements do not allow correction for background radiations caused by external sources, except for the correction for O-activation in the borehole fluid as disclosed in a co-pending U.S. Application Publication No. 2005/0127282 A1. This application by Grau et al. is assigned to the present assignee and is incorporated by reference in its entirety.
In addition to natural gamma ray measurements, some gamma ray tools include a nuclear energy source. Examples include nuclear formation density measurements, which are based on detecting Compton scattered gamma-rays in one or more gamma-ray detectors installed at a suitable distance from a neutron, gamma-ray, or x-ray source. The signals (count rates) detected by such tools are generally much stronger than natural gamma-ray measurements. Therefore, the detectors can be made smaller and, therefore, less sensitive to background effects from extraneous radiations.
Gamma-ray logging tools are often used together with other tools, including neutron tools. When gamma-ray detectors are used together with neutron sources, care must be taken to avoid detecting undesired neutron-induced background signals. To this aim, the gamma detectors are often placed at a distance from the neutron sources. However, it is generally preferred that logging tools be made shorter. As these tools become shorter, the detectors will have to be brought closer to the energy source. There may be situations where the gamma-ray measurements could be influenced by emission from the onboard source, e.g., neutron induced gamma-rays. Currently, there are no methods available to correct for this kind of interference, though correction for borehole fluid and formation activation are disclosed in U.S. Pat. No. 5,459,314, issued to Plasek. This patent is assigned to the present assignee and is incorporated by reference in its entirety.